1. Field of the Invention
The present invention relates to a communication system of a type controlled by polling/selecting.
2. Related Art
Hitherto, data communication systems of information equipment have been characterized as contention systems and polling/selecting systems.
Although the contention system exhibits excellent system extensionability, it must obtain a right to transmit and must perform recovery when communications collide with each other, resulting in a necessity of including complicated hardware and protocol. Even more detrimental, the contention system suffers from a problem that data to be communicated is missed at the time of the collision which deteriorates transmission efficiency.
On the other hand, the polling/selecting system is arranged that its communication flows are concentrically controlled by a control node (a parent node). Therefore, the complicated processes for obtaining the right of communication and the recovery at the time of the collision need not be performed. As a result, it can be realized by relatively simple hardware and protocol.
In the polling/selecting system, the parent node must know the elements (the number of child nodes and the address of each node) of the communication system. Hitherto, the foregoing setting of the elements has been made by an input performed by a user or it has been automatically discriminated by periodic transmission/receipt of a specific message by the parent node.
However, the foregoing conventional technology encounters the following problems when the elements of the communication system are changed, in particular when elements are additionally provided.
First, the user must again input the setting whenever the change has been made in the case where the elements are inputted by the user.
In the case where the parent node automatically discriminates the elements in such a manner that the setting of the elements is automatically made by periodically transmitting/receiving the specific message, the communication traffic accordingly increases, resulting in deterioration in the actual transmission efficiency. In particular, if the periodical intervals are shortened to correspond to the change in the structure, the transmission efficiency deteriorates proportionally. Therefore, the transmissible band range cannot be used effectively.
Another structure may be employed in which the parent node transmits/receives the specific message only when the system is started up. Although the foregoing method is able to prevent the deterioration in the actual transmission efficiency, another problem arises in that the system must be again started up whenever the change is performed.
In the polling/selecting system, the parent node must continue periodic transmission of a message for confirming whether or not data is present at the child nodes in the case where the parent node waits for data supplied from the child nodes. The foregoing case takes place when, for example, a modem is mounted on the child node, and a call from the modem is detected by the parent node followed by performing the ensuing process.
However, the foregoing conventional technology suffers from a problem in that the parent node must continue its operation even if the parent node has no work to be performed except for a work of waiting for data supply from the child node, thus resulting in wasteful consumption of electric power. In the communication system adapted to the polling/selecting system, the parent node cannot be brought into a sleeping state (where the CPU is halted, temporarily stopped, and the main power source is turned off) if there is data to be waited for.
Since the child node is always polled by the parent node, it cannot be brought into the sleeping state although the child node has no data to be transmitted to the parent node.
That is, the overall communication system consumes electric power wastefully.